Implantable, controlled-release osmotic pumps (hereinafter “osmotic pumps”) are known in the art. For example, U.S. Pat. Nos. 3,797,492, 3,987,790, 4,008,719, 4,865,845, 5,057,318, 5,059,423, 5,112,614, 5,137,727, 5,151,093, 5,234,692, 5,234,693, 5,279,608, 5,336,057, 5,728,396, 5,985,305, 5,997,527, 5,997,902, 6,113,938, 6,132,420, 6,217,906, 6,261,584, 6,270,787, and 6,375,978, which are assigned to ALZA Corporation of Mountain View, Calif., and are herein incorporated in their entirety by reference, describe various osmotic pumps. The osmotic pumps described in these references may be designed for implantation in a subject of choice and may be configured to deliver a range of drugs at various rates over predetermined periods of time.
Osmotic pumps typically include a reservoir for containing an amount of drug formulation, an osmotic composition, a semipermeable membrane, a delivery orifice, and a piston separating the drug formulation from the osmotic composition. Upon administration to an environment of operation, water is drawn through the semipermeable membrane of the osmotic pump into the osmotic composition, causing the osmotic composition to swell. As the osmotic composition swells, the piston included in the osmotic pump is driven through its stroke, resulting in the expulsion of the drug formulation at a controlled rate through the delivery orifice. The rate of drug release from an osmotic pump may be adjusted by altering the composition or amount of the drug formulation or the osmotic composition included in the osmotic pump. Alternatively, the release rate of drug formulation provided by an osmotic pump may be adjusted by altering the composition or exposed surface area of the semipermeable membrane. Because they allow the controlled delivery of active agent over periods of weeks, months, or even years, osmotic pumps can advantageously provide long-term dosing of a desired drug without requiring frequent visits to a healthcare provider or repetitive self-medication. Therefore, osmotic pumps can work to provide increased patient compliance, reduced irritation at the site of administration, fewer occupational hazards for healthcare providers, reduced waste hazards, and increased therapeutic efficacy through enhanced dosing control.
As drug formulation is delivered from an osmotic pump, the internal pressure generated by the osmotic composition within the pump generally remains relatively low. However, if an osmotic system is left within an environment of operation after the piston included in the osmotic pump reaches the end of its stroke within the reservoir (e.g., after substantially all the drug formulation has been delivered), the osmotic composition will continue to draw water in from the environment of operation. As water is drawn into the osmotic pump without expulsion of a corresponding amount of drug formulation, the pressure within the system may rise to such an extent that a component of the osmotic pump is compromised or physically separated. Where the semipermeable membrane included in an osmotic pump is held in place through a friction fit, such as is described in, for example, U.S. Pat. Nos. 5,985,305, 5,728,396, and 6,156,331, the semipermeable membrane is one of the components that is most likely to be separated from the osmotic pump if the internal pressure of the osmotic system increases well beyond normal operational pressures.
It would, therefore, be an improvement in the art to provide an osmotic pump that allows the placement of a semipermeable membrane through a friction fit mechanism, yet works to prevent a pressure build-up within the pump that results in the dissociation of pump components, such as the semipermeable membrane. Though not likely to be harmful to a subject, the physical separation of one or more components of an implanted osmotic pump may complicate removal of the device from a subject. Moreover, the physical separation of the semipermeable membrane of an osmotic pump may allow a relatively sudden release of the material forming the osmotic composition, which may result in localized discomfort or inflammation. Thus, where an implantable osmotic pump is designed to dissipate internal pressure before such pressure reaches a level that could cause dissociation of one or more parts, the design of the osmotic pump would ideally allow pressure dissipation without causing a release of osmotic material that results in discomfort or inflammation.